The invention pertains to a process for the preparation of an organic compound by a condensation reaction in which besides the organic compound other products are formed and one or more of these products are extracted from the reaction mixture with the aid of a membrane.
In Japanese Laid-Open publication JP 02/000730 in the name of Nippon Seiroh KK, a process for preparing an ester is described in which the water produced during the reaction is separated with the aid of a water selective separation membrane. The publication discloses membranes based on polyimide or inorganic porous membranes with an average pore size from 0.5 to 5 nm. The preferred inorganic membranes, according to the examples, are membranes with an average pore size of 5 nm, and the separation takes place at 60xc2x0 C. and 75xc2x0 C.
WO 95/07915 and U.S. Pat. No. 5,648,517 also disclose preparative processes in which a membrane is employed to remove one or more of the components formed during the reaction from the reaction mixture.
In general, reactions in which an organic compound is obtained by condensation reactions (monocondensation and polycondensation) are equilibrium reactions. In order to convert a sufficient quantity of the starting material into the organic compound desired in such a reaction type, one or more of the compounds formed are often extracted from the reaction mixture to maintain a favourable overall reaction (high yield of the organic compound desired). In the case of an equilibrium reaction in which water, NH3, methanol and/or ethanol is formed, these products can generally be extracted from the reaction mixture by means of distillation.
Distillation requires that the component to be separated is in the gas phase. Generally, this is achieved by heating the reaction mixture to such a high temperature that the component to be separated from the reaction mixture is removed by boiling. However, such a method has the drawback that, especially when the reaction mixture is viscous, there is bubble formation and foaming, causing additional fouling of the equipment and restricting the degree of filling of the reactor employed. The requirement of boiling limits the degrees of freedom with respect to operation temperature, pressure and choice of reactants. For example, if one of the reactants is very volatile compared to water (for example methanol) or forms an azeotrope with water (for example ethanol) the removal of water via the vapor phase is very inefficient (in particular with respect to reactor use and energy consumption), if possible at all.
With a view of conducting an economical process, it is advantageous to carry out reactions at the highest possible reaction rate. As a rule, it holds that the higher the reaction temperature is, the higher the reaction rate will be. However, the aforesaid foaming or bubble formation frequently makes it impossible to carry out the reaction at such a temperature as will give a sufficiently high reaction rate from the point of view of conducting an economical process. Of course the reaction rate will also be higher if one or more of the products formed during the reaction are extracted from the reaction mixture.
There is a clear need for a process in which the condensation reaction can be carried out at elevated temperature, and the withdrawal of at least one of the products formed during the reaction can be provided in an efficient manner.
The process as mentioned in JP 02/000730 is unsuitable for this purpose, because the polyimide membranes listed are not suited to prolonged use at elevated temperature, and the membranes cease to be active after a while, probably as a result of the membrane becoming clogged. Nor are the membranes of poly(vinyl alcohol) mentioned in the working examples of U.S. Pat. No. 5,648,517 suited to be used at high temperatures.
The active part of the membranes mentioned in WO 95/07915 consists of organic components, such as polyvinylalcohols, and these render said membranes less suitable for prolonged use at elevated temperature.
The present invention as summarized below has overcome the drawbacks of the prior art.
The present invention generally relates to a process for the preparation of an organic compound by a condensation reaction carried out at a temperature above 70xc2x0 C. In said reaction, various other products are formed and such products are extracted from the reaction mixture with the aid of an inorganic membrane with an average pore size of the separating layer of less than 0.5 nm.
According to the invention it has been found that an essential improvement can be achieved by performing the condensation reaction at a temperature above 70xc2x0 C. up to 600xc2x0 C., preferably above 100xc2x0 C. up to 300xc2x0 C., and extracting one or more of the products formed during the reaction by the aid of an inorganic membrane with an average pore size of the separating layer of less than 0.5 nm. Preferably the pores have an average pore size of 0.2 to 0.5 nm.
The reaction-pervaporation process of the invention may be performed in an equipment as illustrated in FIG. 1 and explained below.
Pervaporation tests show that the membrane of the invention has an unexpected high permeation flux of small compounds such as water, especially when considering the small diameter of the pores, but does not allow the reactants to pass through. Furthermore, the membrane has a high ability to maintain its cleanness even after being used for a long time. Cleaning will remain in some cases necessary but the choice of cleaning components and cleaning temperature is less limited than for polymeric membranes.
In addition, inorganic membranes with an average pore size of the separating layer of less than 0.5 nm are particularly suitable for use in heterogeneous reaction mixtures, i.e. mixtures containing both liquids and solid particles. Polymeric membranes generally are not suitable for use in heterogeneous systems, since the solid particles in that case will cause excessive wear of the membranes.
The process according to the present invention may be used in a large number of reactions in which it is favourable to remove one of the products formed particular the removal of water formed during condensation reactions, but also the removal of, for example, methanol, ethanol and/or NH3 may be favourable. It can advantageously be performed in the equipment schematically shown in FIG. 1. The condensation reaction is performed in a reactor 1 and the reaction mixture is pumped via a conduit 2 to a pervaporation unit 3 containing a membrane (not shown). In the pervaporation unit the water formed during the reaction is separated from the rest of the reaction mixture. A vacuum pump 5 and a cold trap 7 facilitate the pervaporation and separation of water, which is discharged through a conduit 6. After the passing of the pervaporation unit the reaction mixture is returned through a conduit 8 to the reactor.
The process is especially suitable for preparing a polymer in a polycondensation reaction, i.e. a reaction in which water is one of the reaction products formed. One example of such a reaction is the formation of a resin, such as an alkyd resin, by reacting a mixture of acids, anhydrides, and alcohols, in which process a resin and water are formed. By extracting water during the reaction, which can be carried out at a temperature between 120xc2x0 C. and 300xc2x0 C., a resin can be obtained in what is a very advantageous manner economically. The comparatively high viscosity of the reaction mixture is no impediment in this case. In case the reaction involves the presence of low boiling compounds, such as methanol, it may be suitable to carry out the reaction at lower temperatures for example between 70xc2x0 C. and 100xc2x0 C.
The process of the invention is also particularly suitable to be used in acetalisation processes for example in the production of glycosides. In such a process compounds containing a hydroxyl group are reacted with saccharides. More specific the organic compound formed can be an alkyl glycoside having the formula
R(OA)n(G)mH
where R is a hydrocarbon group with 1-20, preferably 6-18 carbon atoms, A is an alkylene group having 2-4 carbon atoms and n is a number from 0-10, preferably 0-5, G is a saccharide residue and m is a number from 1-10, preferably 1-5, obtained by reacting R(OA)nH with a saccharide at a temperature from 80-130xc2x0 C., preferably from 90-110xc2x0 C.
Other examples of condensation reactions are esterification reactions between a compound containing at least one carboxylic group and an organic compound containing at least one hydroxyl group. Examples of suitable carboxylic compounds are mono-, di- or polycarboxylic acids and aminocarboxylic acids. Examples of suitable compounds containing at least one hydroxyl group are aliphatic alcohols, phenols, such as nonylphenol; alkylene glycol, polyethylene glycol, propylene glycol, polypropylene glycol, butylene glycol, polybutylene glycol, glycerol, trimetylolpropane, pentaerytritol, saccharides and saccharide derivates as well as alkoxylated products thereof.
The operation of a membrane is based on the fact that the component to be separated has different activities on the two sides of the membrane. Thus, the concentration of the component in the reaction mixture is (comparatively) high, while the concentration of the component on the other side of the membrane, where the component is carried away, is lower.
The membrane may be flat or tubular. The use of a tubular membrane is preferred, since a membrane having such a shape is easy to install in the reaction mixture, the inside of the membrane being closed off from the reaction mixture and the pressure on the inside of the membrane being reduced vis-xc3xa1-vis the pressure in the reaction mixture, for example by connecting the inside of the membrane to a vacuum pump, or with an inert gas flowing through the inside of the membrane. Alternatively, the reaction mixture can be conveyed through the inside of the tubular membrane, while the component to be extracted can be carried away from the outside of the membrane.
The membrane employed in the process according to the present invention has an average pore size of less than 0.5 nm. For optimum service it is preferred to use a membrane with an average pore size of 0.2 to 0.5 nm.
Preference is given to the use of an inorganic membrane, such as a ceramic membrane, due to the mechanical strength and the chemical and thermal stability of such membranes. For example, the membranes can be based on zeolites, xe2x80x9ccarbon molecular sievesxe2x80x9d and amorphous material, such as silica.
Furthermore, membranes, more particularly tubular membranes, can be used to cool or heat the reaction mixture, for example by conveying a liquid having a lower/higher temperature than the reaction mixture through the inside of the membrane. Such a membrane consequently is used as a heat exchanger.
Furthermore, if the membrane possesses sufficient mechanical strength (which is the case, e.g., with tubular ceramic membranes), it is possible to stir the reaction mixture with the aid of the membrane. This is particularly advantageous when the reaction mixture has a higher viscosity or when the reaction mixture is heterogeneous. In such a situation stirring the reaction mixture will also lead to an increased reaction rate.
Since many reactions, for example polymerisation reactions, require a catalyst to initiate and/or maintain the reaction, the membrane can be provided with a catalyst, for example by binding catalytically active particles to the surface of the membrane.
The process according to the invention also offers the possibility of carrying out a reaction as a continuous process, at least a portion of the reaction mixture being conveyed along the membrane, with one or more of the products formed during the reaction being extracted from the reaction mixture. Alternatively, the membrane can be installed in the reaction vessel, in which case one or more of the products formed are extracted from the reaction mixture during the reaction. In such set-ups the reaction can also be carried out in a closed reaction vessel under isochoric process conditions.